Gas monitor using electrochemical cell and method of operating

ABSTRACT

A method and apparatus for gas detection uses a sensor such as an electrochemical (EC) cell and includes a feedback control loop to control a pump to establish a first predetermined gas flow rate to the EC cell. The concentration of the gas at the first predetermined flow rate is measured. If the detected concentration exceeds a predetermined Alert value at the first flow rate an Initial Warning without remedial action is generated, and, the system then changes the gas flow rate before an Alarm is indicated. An Alarm is signaled only if the system verifies the first measurement. Preferably the first flow rate is set to optimize the measurement accuracy of the EC cell being used, and the second flow rate is lower than the first. Verification of an Alarm at the first flow rate may be conducted quickly by a quick-reaction process. The controller may periodically cycle the flow rates between the first and second rates for better accuracy and faster verification times.

RELATED APPLICATIONS

This is a Non-Provisional Application of co-pending, co-ownedprovisional application No. 60/537,087, for “Gas Monitor UsingElectrochemical Cell and Method of Operating”, filed on Jan. 15, 2004and co-pending, co-owned provisional application No. 60/567,140, for“Gas Monitor Using Electrochemical Cell and Method of Operating”, filedon Apr. 30, 2004.

FIELD OF THE INVENTION

The present invention relates to monitors using Electrochemical (EC)cells for detecting and measuring the concentration of gases, typicallytoxic or other undesirable gases, or liquids.

BACKGROUND OF THE INVENTION

Electrochemical cells are used to detect the presence and measure theconcentration of gases or other fluids. Different cells are known to beresponsive to different, specific gases or other fluid/sensorcombinations where the response of the sensor is affected by thepresence of a diffusion layer within the fluid, as with electrochemicalcells. The response characteristic (i.e. voltage or current generated bythe cell vs. gas flow rate) of a particular EC cell to a specific gas isalso known.

EC cells are widely used in industries where toxic or other undesirablegases are present, such as in the manufacture of semiconductors. In suchfields, gas monitoring is continuous and reliable. In the event ofdetection of a gas at a concentration level above an acceptable or safelevel, the monitoring system signals an Alarm and communicates thecondition to operating personnel. Typically, this is followed by ashutdown of the line.

It is known that electrochemical cells generate electrical signals whichare a function of the concentration of known gases. It is also known forsuch cells that the output signal of the sensor is a function of flow,as will be described further within. The signal generated by the cell isnot necessarily a linear function of gas concentration. In addition,however, electrical chemical cells are known to generate electricalsignals (sometimes referred to as background signals or noise) which arenot related to flow rate or the concentration of the gas undersurveillance. That is, for a given gas concentration, EC cells generallyhave a signal component which is flow-dependent, and another componentwhich is independent of or not related to flow rate.

An important feature of the present invention is to take account of orcompensate for the characteristic of electrochemical cells (or othersensors) having a flow-dependent component and a flow-independentcomponent. The flow-independent component may be sensed as related togas concentration and thus cause detection error.

The present invention accounts for the possibility of an appreciableflow-independent component in the output signal generated by the EC cellby changing the flow rate after a first concentration sample, andmeasuring gas concentration at two different but known flow rates, orotherwise using the sensor signal at the second flow rate to confirmthat the concentration measurement at the first flow rate is reliable,and not the result of the flow-independent component of the sensoroutput signal in a manner to be described below.

It will be realized that it could be very expensive to shut down aproduction line, and it is highly undesirable to do so merely because ofan incidental failure of equipment or a non-gas-related event or effectsuch as the presence of background signal related to the sensor cellonly or radio frequency interference (“RFI”), which are known to affectdetection systems. Thus, due to the cost involved in checking orinterrupting a production line, it is highly desirable to avoid thesignaling of false alarms. A fault or background signal may exist in theEC cell itself, or a fault may exist in the monitoring system or be dueto radio frequency interference or transient electrical conditions, andnot necessarily a measure of the concentration of the gas beingmonitored.

SUMMARY OF THE INVENTION

The present invention includes a flow meter to measure the concentrationof the gas being monitored. The monitor, as is known, may draw gas froma larger area under surveillance, and pass the sample through a passageor conduit. A data processor (which may be a microprocessor) monitors aflow rate measurement signal from the flow meter, and controls a gaspump, which in the illustrated embodiment is a diaphragm pump, toestablish a first predetermined flow rate for the gas being sampled.Concentration measurements for a particular known gas or gases are takenperiodically at the first flow rate. If concentration levels are withinan acceptable range or below a predetermined level (referred to as theAlert level), the concentration reading may be displayed and/or recordedwith no further action taken to alert personnel.

The primary purpose of the present invention is to enhance the safety ofusers and personnel within the area being monitored. The present systemaccomplishes this objective by issuing a signal, referred to as anInitial Warning, when a detected concentration C1 of the gas beingmonitored at the first flow rate equals or exceeds the Alert level. Atthis stage, because of the possible errors mentioned above, it is notknown for certain whether the sensed signal is an accuraterepresentative of concentration or, perhaps, due to a system fault orother signal error of the type mentioned. Thus, the present inventionseeks to confirm or deny that the sensed signal causing the InitialWarning is in fact caused by an undesired concentration of the gas beingmonitored or by an error or fault. One immediate benefit provided by theinvention is a higher level of operator confidence in the issuance of anAlarm following an Initial Warning.

To confirm or deny the presence of a detected concentration at the Alertlevel for the gas being monitored, a processor-based controller changesthe gas flow rate to a second, predetermined value. Normally, for mostgases of interest, and particularly where the sensor has adiffusion-limited response characteristic, the second measurement flowrate F2 is lower than the first flow rate F1, but in all cases thesecond flow rate is different than the first flow rate, known andpredetermined by the system.

In a first embodiment of the invention, the gas concentration is thenre-measured at the second flow rate. The gas concentration at the secondflow rate, C2, preferably is proportional to, but at leastrepresentative of the gas concentration (which is known to be flowdependent for most, if not all, EC cells). If the second concentrationmeasurement, C2, confirms that the first concentration measurement, C1,is indeed a matter requiring attention, a suitable notification (e.g. anAlarm) is generated and communicated immediately to operating personnel.If desired, action may be taken automatically by the system. However, ifthe concentration detected at the first flow rate is not confirmed atthe second flow rate, notification is given of the disparity for furtherinvestigation, short of any immediate remedial action or Alarm whichmight cause system shut down.

As used herein, to clarify matters, an “Initial Warning” refers to an“Alert” condition sensed at a first concentration measurement, and itmay be an audible, visual or textual signal, or any combination designedto communicate the sensed Alert condition to operating personnel, butshort of system shutdown. “Alarm” is used to refer to the confirmationor verification signal at the second flow rate, which may be any similarsignal, but also implies that some affirmative remedial action is taken,either by the system itself or by system operators.

The controller has stored in memory, table information specifyingdesired flow rates and concentration data for particular EC cells andassociated gases. Certain EC cells are known to be more accurate whenoperating with gases flowing at specific rates as persons skilled in theart know. In operation, the system may detect the presence in theapparatus of certain, specific EC cells, as by mechanical configurationor keying or electrical identification, alerting the controller to setthe flow rate for optimum sensing capability for the particular cell andgas being monitored, according to the stored table look-up data. Thisenables the same monitoring system to work with different EC cells, andis particularly advantageous to manufacturers of gas monitoring systemswho then need not customize the monitoring system for individual ECcells or for particular gases.

Although it may, in some instances, be acceptable or desirable todetermine the concentration of the detected gas at two different flowrates, it must be realized that what is of concern is the reliabledetection and confirmation of an Alarm condition as quickly as possiblewhile avoiding false alarms, but not at the expense of safety.

Thus, the present invention further contemplates using an optional“quick response” verification procedure or sequence which, upon initialdetection of an Alert condition at the first flow rate, will confirm orverify that the detected Alert condition is in fact being caused by thepresence of the gas and not by sensor error or component failure or someother non-flow-dependent factor such as cell background signal or RFI.The “quick response” verification and Alarm sequence may be completedwithin seconds, not minutes, of the Initial Warning. For example, if asecond concentration measurement is to be completed at a different flowlevel before an Alarm condition is signaled, it may take up to ninetyseconds or more to change conditions, take a reading and make acalculation before remedial action is taken or initiated.

A first quick-reaction verification procedure includes adjusting(preferably lowering) the flow rate of the gas to a second, known flowrate while continuing to measure the signal level of the detectingsensor electronically. It is known that the signal output level of theEC cell is a monotonic (i.e. continuously increasing but not necessarilylinear) diffusion-limited function of the flow rate of the gas. Thus, inthe first quick-reaction verification process, after an initial Alertlevel is reached, the flow rate is changed. If the detector sensing thesignal output level of the EC cell senses a decrease in signal level,with predetermined ranges of magnitude and duration which may bedependent on the particular gas/sensor combination in use, it is takenas a verification that the detecting and measuring circuitry isoperating correctly, that the first measurement is not background fromthe sensor, and that a concentration of the gas above the preset Alertlevel has been confirmed. An Alarm is then generated.

An alternative quick-reaction verification process also changes the gasflow rate after a predetermined concentration of gas Cl at the Alertlevel has been detected at the initial flow rate. The system thenchanges the flow rate to F2 and measures the rate of change and thepolarity of the output signal of the EC cell over a short time. The rateof change must meet predetermined duration and magnitude values,depending on the type of gas being monitored, the type of EC cell beingused, and the two flow rates at which the output signal of the sensor ismeasured or sampled by the system. If the rate of change and thepolarity of the output signal of the EC cell is, for example, negative(in the case of a reduction in gas flow rate), it is a reliableconfirmation that the measuring system and EC cell are operational, thatthe signal is flow-responsive and not cell background noise, and thatthe Alert level of concentration has in fact been exceeded. An alarm isthen generated.

The present invention also contemplates that the flow rate of the gasbeing monitored may be changed on a periodic basis (i.e. cycledrepetitively between F1 and F2), permitting the verification process tobe further expedited by reducing the total lapsed time for the first andsecond sensor measurements. Periodic cycling of the flow rates may alsoimprove the accuracy of the sensor readings at the two flow rates. Forexample, a baseline can be established for the “Alert” level at thefirst flow rate F1 which may improve accuracy and reduce the time forverification.

The quick-reaction verification processes of the present invention areadvantageous because conventional measuring systems typically recorddetected signal levels and such systems are under control of a dataprocessor so that computation of signal levels, polarities and changerates is rapid, straightforward and requires no additional hardware.

Of significance, however, are the following factors: (1) faultverifications incorporating a quick-reaction technique will reduce thetotal time for an Initial Warning measurement and confirmation to amatter of a few seconds, as compared with the ninety seconds or morewhich would typically be required for two successive EC cell signalmeasurements, changing of flow rates and corresponding computations ofgas concentration levels; and (2) the second reading at a different flowrate confirms that the EC cell is measuring a gas response, not sensorbackground signal.

Other features and advantages of the present invention will be apparentto persons skilled in the art from the following detailed description ofvarious embodiments, accompanied by the attached drawing whereinidentical reference numerals will be used to refer to the various stepstaken in the present invention.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic diagram of a gas detection and monitoring systemincorporating the present invention;

FIG. 2 is a diagrammatic view of the flow measuring device of FIG. 1;

FIG. 3 is a diagrammatic showing of the gas pump of FIG. 1;

FIG. 4 is a flow chart of the program for the gas detection andmeasurement algorithm for a first embodiment of the invention;

FIG. 5 is a graph showing the relationship between flow rate and signaloutput for an idealized EC cell;

FIG. 6 is a flow chart showing the operation of the system for a secondembodiment, which includes a first quick-reaction confirmation sequence;and

FIG. 7 is a flow chart of the program for the gas detection system ofFIG. 1 for a second quick-reaction verification sequence embodied in thesystem of FIG. 1.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENT

Referring first to FIG. 1, gas is drawn from a volume being monitoredthrough a filter 4 to a gas detection or monitoring instrument 2, whichmay be referred to as an aspirator. The gas being monitored is fed fromthe filter 4 through an inlet passage 15 to an electrochemical cell 7(“sensor”). The gas is then routed through a passage 3 to a flowmeasuring device (flow meter) 5. The gas is then routed through apassage 3A to a gas pump 6 which returns the gas to the system beingmonitored through an outlet passage 3B. Pump 6 may preferably be aconventional diaphragm pump, having a controllable flow rate asdescribed further below.

The EC cell 7 generates an electrical signal which is coupled along lead16 to a conventional gas detection sensor circuit 10 which detects thesignal of the EC cell 7 and processes it to a digital electrical signalrepresentative of the output of the EC cell 7. This digital signal, inturn, is fed to a processor-based controller 8 which is programmedaccording to a gas concentration measurement and verification algorithm12, below described in connection with FIG. 4. Typically, the gas beingdetected by the sensor is a very small part of the gas volume beingmonitored, but it could be a detrimental or toxic gas.

The flow meter 5 may generate an analog signal transferred to a flowmeasurement drive circuit 9 which generates a digital signalrepresentative of the flow rate of the gas passing through passage 3.The output signal of the flow measurement drive circuit is fed to thecontroller 8, which comprises a data processor (which may be amicroprocessor) programmed according to a flow control algorithm 11 anda gas concentration measurement and verification algorithm 12. Theoutput of the processor 8 from the flow control algorithm 11 is fed to apump drive circuit 14 which, in turn, controls the pump 6 to effect apredetermined flow rate for the gas being monitored.

The flow meter 5, flow measurement drive circuit 9, data processor 8,pump drive circuit 14 and pump 6 form a closed loop feedback controlcircuit to cause the flow of the gas being monitored to a predeterminedrate determined by an operator and controlled by the processor 8. Afirst desired flow rate F1 is stored in the data processor 8 andoptimized for the particular EC cell being used. Specifically, the firstpredetermined flow rate is preferably set to the flow rate at which theparticular EC cell 7 being used, operates at maximum accuracy, which mayvary depending upon the EC cell itself and the gas being detected. Thisinformation is available to users from the manufacturer of the EC cell.By way of example only, and without intending to limit the invention, ifthe gas sought to be monitored is hydrogen sulfide, an EC cell Part. No.MIDAS-S-H2S available from Zellweger Analytics, Inc. of Lincolnshire,Ill. U.S.A. may be used. All necessary operational data is known andpublicly available.

Turning now to FIG. 5, there is illustrated an idealized graph showingthe general relationship, for an EC cell, between flow rate and thegas-responsive (i.e. excluding cell background) signal output. Therelationship is identified in the drawing by reference numeral 65, andit is seen to be a non-linear, but a monotonically increasing function.That is, as the flow rate increases, the output signal of the cell alsoincreases, but the relationship is not linear for all values of flowrate, and in fact the signal begins to approach an upper limit asillustrated. Such relationships between a toxic gas being monitored andthe appropriate EC cell are known in the industry, and are referred toherein as “diffusion limited” response characteristics, referring to thediffusion layer associated with typical electrochemical cells. There area number of toxic gases which can be detected by separate EC cells,although some EC cells have the capacity to monitor more than one gas.

It will also be appreciated from FIG. 5 that for the exemplaryrelationship shown, which is generally representative of many EC cellsof interest, if flow rate F2 is the desired or “optimized” flow rate foroperating the system at a quiescent level of concentration of the gas,and the flow rate is reduced from F2 to F1, the change in gas responsivesignal, from S2 to S1, for the same gas concentration is substantial,and readily detectable, although verification can be obtained byincreasing the flow rate since a differential change in the outputsignal is also experienced for flow rate increases in diffusion-limitedresponse characteristics, as illustrated in FIG. 5.

Referring now to FIG. 2, the flow meter 5 is seen in more detail asincluding a flow restrictor 20 forming a laminar flow of the gas beingmonitored. The gas flows from the passage 3 through a first tee fitting21 to the restrictor 20, and then through a second tee 21A to thepassage 3. A pressure transducer 22 is coupled to the tees 21, 21Arespectively for sensing the pressure difference across the restrictor20. Because the flow is laminar through the restrictor 20, thedifferential pressure sensed by the transducer 22 is representative offlow. The output of the transducer 22 is fed to a flow measurement drivecircuit 9 for converting the electrical signal to an appropriate leveland digital format which is then fed to the data processor 8.

The diaphragm pump 6 is seen in FIG. 3 as including a permanent magnet32 mounted to a diaphragm 30, to which a counterweight 31 is also fixed.The diaphragm 30 has its periphery secured to a frame or housing 36which also houses an electromagnet 33 driven by the flow controlalgorithm 11 of the data processor 8 along lead 18 (FIG. 2). Flowcontrol algorithm 11 generates an output signal along line 18 to thepump drive circuit 14 which controls the pump 6 to operate at apredetermined but controlled flow rate.

Check valves 34 and 35 (FIG. 3) are coupled in series with passages 3Aand 3B of the monitor respectively to insure unidirectional flow of thegas pumped by the action of the pump 6 in the direction of the arrows inFIG. 3. As is known, the magnetic field of the electromagnet 33alternately forces the diaphragm to move periodically away from andtoward the electromagnet 33, thereby alternately expanding andcontracting the volume of chamber 38 defined by the diaphragm 30 and thehousing 36. As the chamber 38 expands, gas flows into the chamber 38through check valve 34. As the chamber 38 contracts under the magneticforce of attraction, gas is forced through the outlet check valve 35into passage 3B. As will be recalled, the data processor 8 is providedwith a table of identifications of specific EC cells and theirassociated gas flow rate for maximum accuracy.

Turning now to FIG. 4, there is shown a flow chart of the gasconcentration measurement and verification algorithm 12 of FIG. 1. Inblock 45, the operator installs a gas sensor cell having tabulatedlook-up data stored in the data processor 8 on the flow rate, accuracyand concentration for that particular sensor or EC cell and theparticular gas under surveillance, an example of which is given above.In block 46, the program sets the flow rate to the detection level, F1,for best accuracy for the EC cell and gas of interest, as determined bythe operator or stored data. In block 47, the gas is detected and avalue is calculated representing the concentration, C1, of the gasdetected by the sensor at a first flow rate, F1. The data processor 8continuously periodically monitors concentration C1 and, in block 48,determines whether the concentration C1 exceeds a predetermined safe setpoint which may be referred to as the Alert level because it has notbeen verified. The Alert level and the Alarm level of concentration arethe same. If the program determines that the predetermined Alert levelis not achieved, the program loops back to block 46 for continuedmeasurement and monitoring. Typically, it takes about ninety seconds ormore to make a complete determination of concentration from the sensedsignals and the known, controlled flow rate, F1.

If, in block 48, it is determined that the concentration C1 does equalor exceed the Alert level, the program issues an Initial Warning to theoperator, and changes the flow rate to a second predetermined level, F2,in Block 49.

The Initial Warning may take many forms, such as audible or visualsignals, or recorded or displayed textual material, or combinationsthereof. However, no remedial action is taken at this time by themonitoring system.

Thereafter, the program enters into a verification procedure, beginningwith block 50 in FIG. 4.

In block 50, after the data processor 8 has changed the flow rate of thegas to a second predetermined level, F2, the value of which is stored inthe data associated with that particular EC cell, the system againmeasures the concentration (C2) of the gas being detected, as indicatedin block 51. Measurement C2 thus represents the concentration of the gasbeing detected at the second predetermined flow level, F2. The secondflow rate F2 preferably may be a lower flow rate than F1, typically lessthan half the first preset flow rate F1. However, persons skilled in theart will appreciate that other flow rates, including greater flow ratesmay be used for the second flow rate setting.

In decision block 53 the system determines whether the concentration C2differs from the concentration measurement C1 by a predetermined amount.This amount depends on factors, including the response characteristic ofthe sensor. A reading indicating that there is no substantial differencebetween concentrations C1 and C2 is taken as an indication that an errorin the measuring system has occurred, such as might be caused bycomponent failure, sensor background signal, subsystem error, RFI orother transient effect. The program, as indicated in block 54, thendetermines that the first measurement, C1, at the first or optimum flowlevel, F1, for present purposes, cannot be verified, and a correspondingmessage is transmitted to the operator, and may be displayed and/orrecorded. Block 54 represents the end of the attempt by the system toprovide further verification of the gas concentration, and the systemnotifies the operator and returns to the main loop at block 51 tocontinue measuring concentration levels.

If, in block 53 it is determined that the second gas concentrationmeasurement C2 at flow level F2 has changed and is different from theoriginal measurement of concentration C1 by at least a predeterminedamount (which is also dependent on other factors) and has changed in theproper direction (if F2 is less than F1, then C2 must be less than C1,not greater), the program determines in block 56 that the firstmeasurement C1 was an accurate or TRUE gas concentration measurement,and the system issues a corresponding ALARM, and may include a notice ofverified gas concentration measurement in block 57.

In FIG. 6, there is shown an algorithm for a first quick-reactionverification process which would provide a verified ALARM condition(and/or take remedial action) in a shorter time than the procedure ofFIG. 4. This algorithm replaces blocks 49-56 in the algorithm of FIG. 4.Turning then to FIG. 6, once it is determined in block 48 of FIG. 4 thata sensor measurement of the EC cell has detected a gas concentration C1in excess of a predetermined Alert set point, an Initial Warning issignaled, and the flow rate is reduced to a predetermined flow rate F2.As indicated above, it is preferred to reduce the flow rate, but theprocess will work if the flow rate is increased in block 61.

While the flow rate is being changed in block 61, the EC cell continuesto read or sample the output signal from the sensor cell, as indicatedin block 62. In block 63, the data processor 8 determines whether thesensed signal output of the EC cell at F2, as determined in block 62,decreases by a predetermined amount after the flow rate has beenreduced. If the output signal of the EC cell does not change, asdetermined in block 63, after the flow rate has been changed to F2 inblock 61, the system determines that the first reading C1 at initialAlert set point determined in block 48 was caused by error, which couldinclude sensor background signal, an equipment malfunction, or anexterior source such as RFI. In this case, the algorithm proceeds alongthe “NO” path 64 in FIG. 6 and ends the first quick-reactionverification algorithm of FIG. 6 to block 54 in FIG. 4, asdiagrammatically represented by the block 65 in FIG. 6.

If, on the other hand, in block 63 the system determines that the sensedsignal output has decreased by a predetermined level in block 63, thesystem takes this indication as a confirmation or verification that theAlert signal which indicated the concentration C1 had exceeded the firstpredetermined Alert set point is, in fact, correct, and the Alert levelgas concentration is confirmed in block 66 and an Alarm generated; andthe operator is notified or remedial action taken automatically or both.The system thereafter changes the flow rate back to F1 in block 67, andagain exits the quick-reaction verification algorithm, but returns toblock 56 in FIG. 4, as indicated by the block 67 in FIG. 6.

The primary advantage of the quick reaction verification algorithm ofFIG. 6, as compared to the verification algorithm of FIG. 4 is that theverification process of FIG. 6 can be completed within a few seconds ofthe original determination that gas concentration exceeds apredetermined Alert set point in block 48, whereas the determination ofconcentration C2 at the second flow rate F2 in blocks 50, 51 of theverification algorithm of FIG. 4, could take as long as ninety secondsor thereabouts to complete.

Turning now to a second quick reaction verification algorithm as seen inFIG. 7, as with the algorithm of FIG. 6, once the system determines thatthe predetermined safe concentration C1, representative of apredetermined Alert set point, is exceeded, an Initial Warning may besignaled to alert the operator, and the flow rate of the gas is reducedfrom F1 to F2 in block 71, while the signal output of the sensor iscontinued to be read by the data processor 8, as represented in block72.

It will be recalled that not only is the signal output stored, but therate of change of the sensor signal is computed and stored by the dataprocessor 8. In block 73, the data processor 8 determines whether therate of change of the output signal of the EC cell exceeds apredetermined amount and the polarity of the change. That is, the dataprocessor 8 differentiates the sensor output signal and determines thepolarity of the change. If, for example, the flow rate is reduced fromF1 to F2 in block 71 and the polarity of the derivative of the outputsignal of the sensor is negative (since the sensor output signal shouldbe reduced when the flow rate is reduced) and exceeds a predeterminedamount as determined in block 73, at the new operating point, it istaken as a determination that the measuring elements of the system areoperative and that the concentration level measured in block 48 isconfirmed in block 74 of FIG. 9. The system signals a confirmed Alarm,then returns the flow rate to its nominal or original value in block 77,and the system returns to block 56 in FIG. 4. If, on the other hand inblock 73, after the flow rate is reduced to F2, the derivative of thesensor output signal does not both exceed a predetermined amount andhave the required polarity as determined in block 73, then the system sonotifies the operator without signaling an Alarm, and returns to block54 in FIG. 4.

The quick reaction verification algorithm of FIG. 9 will reach adetermination of a confirmed Alarm condition in substantially less timethen that of FIG. 4 because determining the rate of change of the sensorsignal and the polarity of the signal change in block 73 by the dataprocessor 8 can be done in a very short period of one second or less,and it can then be compared with a predetermined level of rate of changeeven more quickly. Again, the verification algorithm of FIG. 9 takes asubstantially less time than that of FIG. 4 because it does not requirethat the verification algorithm determine the concentration of the gasbeing monitored at flow level F2.

The nature of the visual or audible signals, displays or recordings forthe Initial Warning and Verified Alarm conditions preferably aredifferent so that the operator may differentiate them and immediatelydetermine the status of the monitoring system.

To further reduce the verification time in any of the three methodspresented above as well as to increase accuracy, it may be desired tocontinuously cycle the flow rate between F1 and F2. Once a measurementfrom the sensor 7 is taken, the concentration must then be determined bythe data processor 8 according to known procedures, and thisdetermination takes time.

In an alternative embodiment, the data processor 8 changes the flow rateto F2 as soon as the sampled signal from the sensor is received by thedata processor and while the computation and determination of C1 isbeing made. The confirmation of block 66 of FIG. 6 will thus be soonerin time because the sensor measurement at F2 may be completed or nearcompletion by the time concentration C1 is determined.

Moreover, by periodically cycling the flow rate of the gas, the dataprocessor can make measurements of the average or quiescent values ofsensor signal output at F1 and F2, determine and employ averaging andother statistical techniques to determine and compensate for noise andbackground signal, determine a baseline signal for concentration levelsand flow rates, and provide other information.

The two phases of the periodic cycling need not be of the same timeduration depending on the verification technique used. Further, if oneof the quick-response verification techniques described above isemployed and the flow rate is cycled, the duration of the signal forcingpump 6 to operate at flow rate F2 may be considerably shorter thanoperation at flow rate F1, and a verification determination may be madealmost immediately after the computation of C1 is complete, thusshortening the overall time required to complete a measurement andverification cycle.

Having thus disclosed in detail a number of embodiments of the presentinvention, persons skilled in the art will be able to modify certain ofthe steps which have been disclosed, and to substitute equivalentcomponents or structure for that which has been described; and it is,therefore, intended that all such modifications and substitutions becovered as they are embraced within the spirit and scope of the appendedclaims.

1. A method of confirming the presence of a predetermined concentrationof a detectable gas in a gaseous mixture comprising: flowing the gaseousmixture at a predetermined first flow rate in contact with a sensor togenerate an electrical signal representative of the concentration ofsaid gas; determining from said electrical signal the concentration C1of said gas; then comparing said concentration C1 with a predeterminedvalue representative of an Alert level of concentration of said gas;generating an Initial Warning when said concentration of said gas is atleast as great as said Alert level; changing the flow rate of saidgaseous mixture to a second flow rate different from said first flowrate; and generating an Alarm if said electrical signal indicates aftersaid flow rate has been set to said second flow rate that said InitialWarning was a reliable indication of the concentration of said gas atsaid first flow rate.
 2. The method of claim 1 wherein said step ofgenerating an Alarm comprises: determining the concentration C2 of saiddetectable gas at said second flow rate; comparing concentrations C1 andC2; and thereafter generating said Alarm when said concentrations C1 andC2 differ by at least a predetermined amount.
 3. The method of claim 2wherein said step of changing the flow rate of said mixture comprisesdecreasing said flow rate to said second signal predetermined flow rate;and determining that said second electrical signal representative of C2is less than said electrical signal representative of C1.
 4. The methodof claim 1 wherein said electrochemical cell has a flow versus outputsignal characteristic which is monotonic and has a flow-limited rateresponse characteristic.
 5. The method of claim 1 wherein said step ofgenerating an Alarm comprises determining whether said electrical signalof said sensor has changed by at least a predetermined amount from thesignal output of said sensor at said first flow rate; and thereuponsignaling an Alarm in confirmation of said Initial Warning when saidsignal output of said electrochemical cell has changed by at least saidpredetermined amount at said second flow rate.
 6. The method of claim 5wherein said step of changing flow rate comprises decreasing the flowrate, and further comprising the step of determining that said secondelectrical signal has decreased from said first electrical signalrepresentative gas concentration at said first flow rate.
 7. The methodof claim 1 wherein said step of generating an Alarm comprises:continuing to measure the signal output of said sensor after changingsaid flow rate; determining the rate of change of said signal output ofsaid electrochemical cell after changing to said second flow rate;determining the polarity of signal change of said output signal of saidsensor after changing to said second flow rate; and generating saidAlarm when the magnitude of the rate of change of said signal output ofsaid electrochemical cell is greater than a predetermined amount andsaid polarity corresponds with the direction of change in flow rate. 8.The method of claim 6 further comprising: determining the polarity ofsaid rate of change of said signal output to confirm a sensed Alertlevel before signaling said Alarm.
 9. The method of claim 1 furthercomprising the step of periodically cycling said flow rate between twopredetermined rates.
 10. Apparatus for detecting and confirming theconcentration of a known gas in a gaseous mixture at least as great as apredetermined Alert level of concentration, comprising: a conduit forcoupling to said gaseous mixture; a sensor for detecting said known gasin said passage and generating a signal representative of the amount ofsaid known gas present in said mixture; a pump having a controllableflow rate pumping said gaseous mixture through said passage at acontrolled flow rate; a controller including a programmed data processorreceiving said signal of said sensor for determining the concentrationC1 of said known gas at a first flow rate determined by said controller;said controller programmed to determine when said concentration is atleast as great as an Alert level, and thereupon to generate an InitialWarning to operating personnel, said controller thereafter causing saidpump to operate at a second flow rate different from said first flowrate whereat said controller continues to receive the output signal ofsaid sensor to confirm whether said concentration measurement at saidfirst flow rate was affected by error, and to signal an Alarm when it isconfirmed that said first confirmation measurement was not erroneous.11. The apparatus of claim 10 wherein said processor is furtherprogrammed to compute said concentration of said gas at said second flowrate and to compare said concentration measurements to confirm or denysaid first concentration measurement.
 12. The apparatus of claim 9wherein said processor is programmed to periodically cycle said flowrate between two predetermined flow rates.
 13. The apparatus of claim 10wherein said processor is further programmed to receive said outputsignal of said sensor after said flow rate is changed to said secondlevel and to determine the amount of any corresponding change in themagnitude and polarity thereof to confirm or deny the accuracy of saidfirst concentration at said Alert level.
 14. The apparatus of claim 13wherein said controller controls said pump such that said second flowlevel is lower than said first flow level.
 15. The apparatus of claim 10wherein said controller continues to receive said output signal of saidsensor after said change in flow rate and determines the magnitude andpolarity of such change and therewith confirms or denies the validity ofsaid first concentration measurement.
 16. The apparatus of claim 10wherein said controller controls said pump such that said second flowlevel is lower than said first flow level and said controller isprogrammed to determine that said sensor output signal decreases whensaid flow rate is decreased by said controller.
 17. The apparatus ofclaim 10 wherein said sensor is an electrochemical cell.
 18. Theapparatus of claim 10 wherein said sensor is characterized as having aflow limited response characteristic.
 19. The apparatus of claim 10wherein said processor is further programmed to control said pump torepeatedly periodically cycle said flow rates between said first andsecond flow rates.